The present invention relates to a device for boosting and venting a gas turbine engine such as a gas turbine jet engine used for aircraft propulsion said engine being of the kind comprising at least two independent rotating stages, namely a low-pressure rotating stage comprising a low-pressure compressor, and a high-pressure rotating stage comprising a high-pressure compressor, said engine comprising a primary duct through which, in operation, there flows a primary air flow passing successively across the low-pressure compressor and the high-pressure compressor, said primary duct being separated by a casing, from an air space located outside the duct.
The boosting and venting device is of the kind which comprises:
AT LEAST ONE MAIN DUCT STARTING IN THE PRIMARY DUCT SECTION DEFINED BETWEEN THE LOW-PRESSURE COMPRESSOR AND THE HIGH-PRESSURE COMPRESSOR, AND ESTABLISHING COMMUNICATION, ACROSS SAID CASING, BETWEEN SAID PRIMARY DUCT SECTION AND SAID AIR SPACE; AND
AT LEAST ONE MAIN SHUTTER ELEMENT, MOVABLE IN RELATION TO SAID MAIN DUCT AND CAPABLE OF OCCUPYING AN OPEN POSITION, IN WHICH SAID MAIN DUCT IS OPEN, AND A CLOSED POSITION IN WHICH SAID MAIN DUCT IS CLOSED.
The present invention relates more particularly, albeit not exclusively, to the case in which said gas turbine engine is of the dual-flow kind. The air space located outside the primary duct is then constituted by a secondary duct through which, in operation, there passes a secondary airflow coming from a fan secured to the low-pressure stage for rotation therewith, said fan being equipped, if required, with variable-pitch blades which are capable of going into reverse pitch.
The invention seeks in a general way to improve the operation of the engine in various critical operating phases: starting and acceleration of the engine, and braking of the aircraft by reversal of the fan-blade pitch.
It will be remembered, in this context, that the starting of dual-stage, dual-flow gas turbine engines (in particular engines in which the dilution ratio, that is to say the ratio between the secondary and primary airflows, has a high value), presents major difficulties. The low-pressure stage of these engines, which is equipped with a large-sized fan, is in other words difficult to start especially if, in the presence of a wind from the rear, the fan is already rotating in a direction which is the reverse of its normal direction of rotation. When the high-pressure stage is started, a certain amount of time consequently elapses before the low-pressure stage (and therefore the low-pressure compressor) starts to rotate in the proper direction. During all this time, the airflow available at the high-pressure compressor input, is very low and there is the possibility, on the one hand, of the compressor stalling, and on the other, of dangerous overheating taking place.
A phenomenon of this same sort occurs if, in order to produce aerodynamic braking of an aircraft by reversing the direction of the airflow through the secondary duct, the pitch of the fan-blades is reversed. In this case, too, the high-pressure compressor runs the risk of being starved.
In both the aforementioned instances, it can therefore be of interest to short-circuit the low-pressure compressor and to directly introduce fresh air to the high-pressure compressor input; this is the technique known as super charging or "boosting".
Conversely, it may happen that in order to avoid the possible risk of pumping of the low-pressure compressor, it is necessary to feed across this compressor an airflow which is in excess of that which can be absorbed downstream. The excess air must therefore be bled off at the compressor output. This is the technique known as compressor "bleeding".
Boosting and bleeding thus each employ the presence of a passage which, at the desired instant, will provide communication between the primary duct and an air space located outside said duct. It should be pointed out in this context that the cross-section of the bleed duct should be much smaller (in view of the relatively high pressure of the air which is to be bled off) than that of the boosting duct.
The present invention proposes a combined boost and bleed device, which is simple, light, relatively compact and inexpensive, capable of alternately performing the two functions, namely boosting the high-pressure compressor (during the phase of starting the engine or reversing the fanblades) and bleeding the low-pressure compressor (during the phase in which the engine is accelerating).